1. Field of the Invention
The present invention relates to a connecting rod for an internal combustion engine, and further, to methods for producing the connecting rod.
2. Description of the Background Art
Connecting rods are used for connecting the pistons of an engine to the crankshaft. Connecting rods are provided with a small end section and a large end section formed in a continuous monoblock at the two ends of the rod section. The large end section is divided into a body side and a cap part, and is then subjected to a surface hardening treatment process. Thereafter, the body side and the cap part are assembled together.
This assembling step is accomplished either by joining the cap part to the body side with bolts and nuts (hereinafter called the bolt-nut joining type; see Utility Model Laid-Open No. Sho 60-12718 for example) or by providing female threaded holes in the body side and joining the cap part to the body side with threaded bolts (hereinafter called the nutless type).
FIG. 6 is a diagram showing a partial sectional view of the large end section of a conventionally adopted bolt-nut joining type. As shown in FIG. 6, after the large end section is divided into a body side 1 and a cap part 2, throughholes 5 and 6 are bored respectively in seat sections 3 and 4. Then, the divided parts are subjected to a surface hardening treatment process, and then the cap part 2 is joined or assembled with the body side 1 with bolts 7 and nuts 8.
FIG. 7 is a diagram showing a partial sectional view of the large end section of a nutless type, also conventionally adopted, wherein joining is accomplished only with bolts 7 by providing female threaded holes 9 in the body side 1.
In FIG. 6, the seat section 3 on the body side 1 constitutes a shoulder part protruding sideways in the bolt-nut jointing type. However, no such seat section is formed on the body side 1 in the nutless type of FIG. 7, and therefore, the overall shape substantially forms a sloped shoulder.
In the bolt-nut joining type shown in FIG. 6, a considerable thickness portion A is provided near the seat section 3 so that this area does not become the weakest part in strength. For this reason, the thickness A near the seat section 3 is increased, causing the area of the seat section 3 constituting a shoulder to protrud sideways. This increase makes it difficult to achieve weight reduction of the whole connecting rod.
On the other hand, in the nutless type shown in FIG. 7, it is necessary either to form the female threaded holes 9 before the surface hardening treatment process, and then subject them to the surface hardening treatment process after an anti-carbon treatment process, or to form the female threaded holes 9 after subjecting them to the surface hardening treatment process. However, in the former case, the man-hours required for the anti-carbon treatment process increases and moreover, secure anti-carbon treatment is difficult. In the latter case, since the threads are machined into the surface hardening-treated layer, such machining becomes extremely difficult due to the high strength of the surface hardening-treated layer.
For these reasons, in both cases, the man-hours required to form the connecting rod increases. Moreover, difficult machining is inevitable. Therefore, a nutless type connecting rod which realizes weight reduction and which permits female threaded holes to be easily formed is desired. Further, a production method for producing such a connecting rod is desired.